Canine distemper virus (CDV) infection with resulting demyelinating encephalomyelitis in dogs will be used as an animal model of subacute sclerosing panencephalitis and possibly other demyelinating diseases of man, such as multiple sclerosis. In vitro studies will focus on production of persistent CDV infection various tissue culture cell lines from different species, selection of temperature sensitive mutants produced in these cells and testing of their pathogenicity in dogs. Production of interferon by different virus populations, and their susceptibility to it, will be tested. Studies in dogs will include a search for factors that might induce persistent infection such as interferon, autointerference, viral induced immunosuppression, immune complexes, or masking of antigen on the infected cell surface.